Wireless access technologies experience transitions from one generation to another. During these transitions, the market typically requires that both old and new technologies be supported so that users of the old technology can operate in parallel at the same network with users of the new technology. Such a requirement creates problems both to operators, who need to replicate costly infrastructure, and moreover, need to divide the frequency band which they have licensed, into one sub-band accommodating transmissions that use the old technology and another sub-hand accommodating the transmissions using the new technology. Such rigid partitioning implies inefficient utilization of the resources as the usage pattern of the technologies varies over time. From the equipment manufacturer perspective, the introduction of new technology, even if it has not yet matured, impedes the development of the existing technology due to market expectation.
An example at hand is the transition of IEEE 802.16™ from purely fixed services to combined fixed and mobile operation. The international standard includes three physical layers—referred to as PHY—Single Carrier, OFDM and OFDMA, of which, the OFDM mode has been chosen by the WiMAX forum as the technology of choice for fixed deployments. For the combined fixed and mobile mode of operation, which at present is being developed as a recommendation to be issued by the 802.16e committee, the industry perceives OFDMA technology to be the preferred technology. This preference might impede the deployment of the OFDM technology even though mobile, or combinations of fixed+mobile WiMAX systems, might still be few years away.
Solutions for such technology transition were discussed in the past. In their “Coexistence of Fixed and Mobile Services”, IEEE C802.16e-03/02 (January 2003), the authors Avner Alush, Marianna Goldhammer, Vladimir Yanover, proposed a frame structure which shares the airframe between fixed and mobile services in the time domain. This proposal achieves the target of having flexible resource allocation between the fixed and mobile parts of the frame. However, the main drawback of this proposal is that the mobile sub-frame does not start at periodic intervals and each MAC frame has a duration or start time according to that announced in its preceding MAC frame—a characteristic which complicates synchronization of mobile transmissions. Moreover, the mobile subscribers are assumed to receive the “fixed” portion of the downlink, which in general is less robust than the mobile part. Another proposal “Harmonization of 2K OFDMA and 256 Sub-channalization”, ETSI BRAN 33.5, Leuven, 12-14 Aug. 2003, was made by Runcom to the HIPERMAN committee and presents an idea in which both the OFDM and OFDMA frames are interleaved so that each subscriber may synchronize on the transmissions of the technology of choice while ignoring the other transmissions. The limitation of this concept is that the percentage of time, and consequently the data traffic allocated to each of the technologies, cannot be changed dynamically.
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